Both very good, but with different qualities.
Tempestas are much larger (twice the volume) and have deeper bass. Midbass through the upper mids are very similar. The RAAL ribbon has the ribbon sound, whereas the top end on the Merlins is dome like-less air, but more body.
Merlins are more spacious sounding, Tempestas drier sounding.
In my room I need a lot of bass energy- it really eats bass. So for me, the Tempesta is preferred. In a smaller room, or with tubes, the Merlins might be preferred.
My Tempestas were also more than 3x the price as well.
